Hard to word that any better.
I had a long post written out Sunday morning, but it was a bit pissy/preachy and not of much help to anyone, so I deleted it.
Verify there is a problem, some engines ping and thats all there is to it. You may think it is irregular, but FoMoCo, in order to meet CAFE standards, may think that pinging is OK. Techs call it "normal operation" or NPF (no problem found).
See if there are any factory TSBs or service bulletins pertaining to your problems. A competent tech should have checked, but sometimes they get in a hurry, are lazy, or a combination of the above. There could be a PCM recalibration update that addresses your concern. A competent tech will ck for codes, go into DCL and pull up fuel trims, ECT, DPFE voltage and take it on a test drive in order to duplicate the problem. How far was the vehicle driven by the tech while trying to diagnose it? If the "mileage in" and "mileage out" are the same on the RO, someone didn't test drive it.
Since this is an ongoing concern (2 dealer visits) the dealer should be able to either fix it or flat out tell you the operation (ping) is normal without addl charges. If they can't get it to ping, if possible drop by when it acts up, drive it with a tech, and have him hear the problem to verify the concern. I have had customers take me on test drives in the past, and it cleared up a lot of miscommunications/bad writeups by service writers. Some customers just had "one of a kind" driving habits/situations that amplified a one in a million concern.
Fixing the symptoms by going to colder plugs or especially installing a cooler thermostat is just a bad thing. Get it fixed right.
Sermon over.